1. Field of the Invention
This invention relates to a mounting structure that an electronic component is mounted on a flex-rigid board where the electronic component has a lead interval narrower than the thickness of the flex-rigid board, and relates to an optical transceiver using the same.
2. Description of the Related Art
As shown in FIG. 1, an LD (a semiconductor laser) element module 133 (e.g., a package with LD or an LD sub-assembly) may be mounted on a rigid-flex board (or flex-rigid printed circuit board) 132, where the LD element module 133 has a lead interval w2 (w1>w2) narrower than the thickness w1 of the rigid-flex board 132. Also, a PD (a photodiode) element module (e.g., a package with PD or a PD sub-assembly) may be mounted
The board 132 is composed of a rigid board 132r and a flexible board 132f, which are laminated alternately. The LD element module 133 is mainly composed of: a can package 134 that is made of metal with a high heat radiating property and has an airtight inner portion; an LD element that is placed in the can package 134 and emits an optical signal; a lens 135 that collects light emitted from the LD element; and plural leads 136.
Several methods of attaching the lead 136 to the board 132 are assumed as follows.
(1) As shown in FIG.2, the first is an electronic component mounting structure 141 that the lead 136 subjected to the lead forming (being bent in part) is attached onto one surface of the board 132.
(2) As shown in FIG.3, the second is a method that the lead 136 is attached to the board 132 while sandwiching the board 132.
(3) As shown in FIG.8, the third is an electronic component mounting structure 201 that an IC 202 is mounted on the board 132. The IC 202 has plural leads 203 with leads extending linearly in the horizontal direction. Therefore, the leads 203 need to be subjected to the lead forming when they are attached onto the board 132.
The related arts of the invention are disclosed in, for example, Japanese patent application laid-open Nos. 2004-71890 and 2003-209294.
However, the first method (1) and the second method (2) have problems as described below.
Namely, (i) since in the first method (1) the lead 136 is subjected to the lead forming, it takes time and a trouble to form the lead.
(ii) The second method (2) where the board 132 is sandwiched by the leads 136 allows the LD module 133 to be attached more securely than the first method (1) where the leads being all subjected to the lead forming are attached onto one surface of the board 132. However, in the second method (2), a stress may be collected in the lead 136 to support the can package 134 to reduce the reliability of the lead 136 and the board 132.
Further, in the second method (2), since the thickness w1 of the board 132 is greater than the lead interval w2, the lead 136 may collide with the board 132 as shown in FIG. 4 when the lead interval w2 is kept as it is.
To solve the problems, a structure may be assumed that the board 132 is sandwiched by elongated leads 136. For example, as shown in FIG. 5 and FIG. 7, which is a plain view corresponding thereto, an electronic component mounting structure 171 is assumed that the leads 136 are attached thereto while being forcedly bent and spread. Further, as shown in FIG. 6 and FIG. 7, which is the plain view corresponding thereto, an electronic component mounting structure 181 is assumed that one of the leads 136 is subjected to the lead forming on the back side of the board 132.
However, the mounting structures as shown in FIGS. 5 and 6 have problems as described below. When an RF signal (e.g., 5 GHZ or more) passes through the lead 136, the high-speed signal may deteriorate due to the elongation of the lead 136. In other words, a gap L1 must be generated between the end of the board 132 and the can package 134 and, therefore, the lead 136 cannot be mounted in the shortest length on the board 132, i.e., the LD element module 133 cannot be mounted in the shortest distance on the board 132.
Further, when the leads 136 are forcedly bent and spread as in the mounting structure 171, an excessive stress is applied to the base of the lead 136 and, thereby, the airtightness may be broken between the lead 136 and the can package 134.
On the other hand, the mounting structure 201 as shown in FIG. 8 has problems as described below. Since the leads 203 are subjected to the lead forming, the problems (i), (ii) as described earlier are generated and, further, when the RF signal (e.g., 5 GHz or more) passes through the lead 203, the high-speed signal may deteriorate at the bent portion of the lead 203, thus causing deterioration in the RF characteristics. Also, since the heat radiation is not considered, the structusre will suffer from a low heat radiating property.